The present invention relates to a safety catch device for blocking movement of a rod relative to a component of a mechanism with respect to which the rod is normally movable, which device includes clamping means surrounding the rod, and a compression spring disposed to produce a force which acts on the clamping means to clamp the latter against the rod, the clamping means being releasable from the rod clamping position against the action of the compression spring.
In a prior art safety device of this type, such as disclosed in German Pat. No. 2,333,491, clamping bodies in the form of a plurality of brake jaws are disposed adjacent one another in the circumferential direction and surround the rod in the form of circular sectors. On their side adjacent the jaw they have a partially cylindrical inner face which is matched to the diameter of the rod. At their opposing outer side they each have a conical shape and are axially displaceably mounted in a housing ring having a corresponding inner cone. Since the conical faces of the brake jaws of the housing ring fit together without leaving gaps only when in a single position, when there is congruence of their diameters, while in another, lower or higher, position of the brake jaws there result stresses on the edges with unduly high compressive pressures, the brake jaws must each be provided with guide faces which are inclined so as to correspond to their conicity but are planar in the circumferential direction and move over a needle cage at an identical guide face in the inner cone of a housing ring. These guide faces must be manufactured with extreme precision so that surface contact is produced between the brake jaws and the rod.
Due to the conical faces at the brake jaws and the housing ring, and particularly due to the oblique but inherently planar guide paths along the conical faces, the manufacture of these prior art safety catches is rather expensive. Moreover, there does not exist a defined setting force which keeps the brake jaws out of contact with the rod when they are not in the clamping position. Finally, because of the required longitudinal gaps between the individual brake jaws it is very difficult to exert an axially symmetrical clamping pressure, which is a prerequisite for uniform gentle treatment of the friction partners.
Although German utility model Pat. No. 1,895,972 discloses a structure which eliminates the structurally complicated guide paths in the conical faces, the above-mentioned surface pressure conditions and, in addition, considerable friction forces are unavoidably encountered during axial displacement of the brake jaws. The high manufacturing costs for the conical parts remain the same.
Finally, German Auslegeschrift [Published Application] No. 1,180,921 discloses a safety or clamping device for hydraulic or pneumatic lifting platforms. This device includes a resilient, conical clamping ring which encloses the lifting column and is enclosed, through the intermediary of a resiliently supported ball cage, by a clamping bell. A clamped position of the conical part can be realized with one manual setting. This apparatus is also relatively complicated structurally and, in addition, can be integrated into existing lifting systems only with difficulty since it occupies a relatively large space. Moreover, it permits only slight clamping forces to be applied.